1. Field of the Invention
The invention relates to gain control of an optical modulator. In particular, although not exclusively, the invention relates to dynamic gain control for use with control loops of a Mach-Zehnder modulator.
2. Description of the Related Art
Transmission of data using optical carriers enables very high bandwidths and numbers of multiplexed channels with low signal loss and distortion. A coherent laser light beam is amplitude or phase modulated with a data signal, and propagates to a remote receiver via a system of optical fibres and repeaters. The light beam may advantageously be modulated with electrical signals in the microwave frequency range using an electro-optic modulator such as a Mach-Zehnder modulator or optical coupler.
Mach-Zehnder electro-optic modulators are well known. An electro-optic modulator based on a Mach-Zehnder interferometer generally includes a monolithic substrate formed of an electro-optic material such as LiNbO3 or InP. An optical waveguide is formed in the substrate having two arms or branches which extend generally in parallel with each other. The effective index of refraction of the material in the waveguide is higher than the index of refraction of the material of the substrate.
In the absence of an applied electrical bias voltage, an input optical or light beam produced by a laser or the like applied to the waveguide divides between the branches. The optical signals propagating through the branches recombine at the optical output of the waveguide. If the optical path lengths of the branches are equal, or differ by an integral number of wavelengths, then the optical signals recombine in phase with each other, such that their amplitudes are additive and an optical output signal which is essentially similar to the optical input signal appears at the output of the waveguide.
If the optical path lengths of the branches differ by a half integral number of wavelengths, the optical signals emerging from the branches are 180° out of phase with each other. The amplitudes of the signals combine subtractively, cancelling each other out, such that a zero output is produced at the optical output.
Application of a predetermined electrical bias voltage differential to one branch of the waveguide relative to the other branch causes the complex indices of refraction of the material in the branches to vary differently due to the electro-optic effect, such that the effective optical lengths (and absorption) of the branches vary accordingly. At a bias voltage known in the art as Vπ, the effective optical lengths have varied to such an extent that the optical signals emerging from the branches are 180° out of phase compared to the situation when no bias voltage is applied. If the path lengths of an unbiased modulator are the same, then at a bias voltage of Vπ the optical signals will interfere destructively and cancel each other out, such that a zero output is produced at the optical output. If the path lengths of an unbiased modulator differ by a half integral number of wavelengths, then at a bias voltage of Vπ the optical signals will interfere constructively so that the optical output signal is essentially similar to the optical input signal. An electrical data signal, normally in the microwave frequency range, is applied to one or both of the arms. The optical carrier signal exiting the device is thus modulated by the data signal.
Phase-shift keying (PSK) is a digital modulation technique that conveys data by changing (i.e. modulating) the phase of a carrier signal instead of its amplitude. Essentially, binary digits (bits) are encoded by associating a discrete set of phases of the carrier signal with a particular pattern of bits, known as a symbol. In differential phase-shift keying (DPSK) it is the change in successive phases of the signal that is used to determine the bit pattern, rather than the actual phase of the signal at any point in time.
Binary phase-shift keying (BPSK) makes use of two distinct phases separated by 180° . However, this technique only allows for 1 bit to be encoded per symbol and so it is not suitable for high data-rate applications. Quadrature phase-shift keying (QPSK) employs four discrete phases and can be used to encode two bits per symbol by combining an in-phase wave and a quadrature-phase wave, having a phase-shift of a quarter of a wavelength with respect to the in-phase wave. Thus, QPSK can be used to significantly increase a data rate when compared to BPSK. Both BPSK and QPSK can be implemented using differential PSK to form differential BPSK (DBPSK) and differential QPSK (DQPSK), respectively.
For many optical communication applications, it is desirable to bias modulators at an “operating point” voltage Vπ/2. However, device instabilities and environmental effects, especially temperature variations, cause the operating point to drift over time, and constant readjustment is required to maintain the proper operating point. When MZMs are used to encode PSK schemes, they are required to manipulate the phase of the light exiting the modulator without substantially affecting the amplitude. When this is the case, each MZM is biased for minimum optical transmission in the absence of a drive voltage (i.e. the operating point voltage is Vπ), and is driven with a drive voltage VI(t), VQ(t)=±Vπ to give abrupt phase shifting with a minimum of amplitude modulation.
MZMs typically used for PSK schemes generally have control loops relying on phase control transfer functions which are non-linear. They typically rely on integrated “imbalance” control electrodes within their control loops to centre the operating point of an optical signal. An imbalance electrode is integrated into each MZ arm. The imbalance electrodes can be operated single ended or differentially.
Phase change in the optical signal is achieved by injecting current into the imbalance electrode. This effectively achieves the same result as applying a change in bias voltage on a bias electrode (as described above). The amount of optical phase change per unit current (mA) is non linear. In general, the change in optical phase per mA is greatest at the lowest injection current. As the current is increased from zero, the phase change initially increases sharply with current, but as the current continues to rise the rate of change of phase change decreases.
Operation of the control loop relies on measuring an “error” introduced by an imposed dither in the optical phase. The optical phase is caused to oscillate at a predetermined rate (which is much lower than the bit rate of the transmitted signal). In order to impose the dither on the optical phase, a dither signal is applied to the bias voltage or to the current injected into the imbalance electrode. The amplitude of the dither signal is generally fixed.
The output signal from the modulator is monitored and a feedback loop adjusts the current applied to the imbalance electrode so as to correct the phase to ensure that the modulator remains at the operating point. The magnitude of phase correction in terms of current applied to the imbalance is a product of a fixed gain and the error magnitude and is therefore linear.
However, the magnitude of optical phase correction for an arbitrary change in current is not uniform for all currents injected into the imbalance electrode. The current change required to achieve any given phase correction will vary across the imbalance characteristic and is therefore non-linear. As a result, the overall loop gain will change across the characteristic. The highest gain will occur at the lowest imbalance current. It is possible that operation and transition at or across zero imbalance current may be required and there is a danger that control may be lost when the imbalance current is low.
One way to address this problem is to apply a small current offset (for example 150 μA) to the imbalance electrode on both arms. This ensures that the current is never too close to zero and “soaks up” the rapid change in phase at the lowest imbalance currents. However, a range of phase control is lost by following this route.